The Universal Mobile Telecommunication System (UMTS) is one of the 3G mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. The complete cellular system that comprises an LTE system (and thus E-UTRAN) is denoted Evolved Packet System (EPS). A radio access network typically comprises user equipments (UE) wirelessly connected to base stations (BS), commonly referred to as NodeB in UTRAN and eNodeB in E-UTRAN. Each BS serves one or more areas each referred to as cells.
One of the hot topics for future developments of cellular systems is heterogeneous networks. Heterogeneous networks are networks with a mixed deployment of cells of widely different sizes, such as macro cells, micro cells, pico cells, and femto cells. The cells may either cover overlapping areas, e.g., when the area covered by a pico cell is also covered by a macro cell, or they may complement each other's coverage. Smaller cells, such as pico cells, will typically be overlapping with other cells. They will be used, e.g., to provide increased capacity at locations with dense user populations and high traffic volumes, so called hotspots, or to provide improved coverage, e.g., in terms of better channel quality at certain indoor locations.
Deploying small base stations to enhance capacity or coverage seems to be an appealing way for operators to overcome the cumbersome of finding and leasing new macro sites. The main characteristics of these small base stations are that they are smaller in size compared to a typical macro site, easily installed, e.g. they can be attached to lamp posts, walls, etc., and their transmit power is often lower compared to the transmit power of a macro site.
One issue with the deployment of these small base stations is at what extent they can offload macro sites or how large areas they can cover. Simulations with various channel models have shown that this coverage area is typical small to medium compared to the coverage area of a typical macro site. Additionally, the benefits of cell-area splitting gains are usually marginal and deployment of the small base stations does not pay off. One way to increase the coverage area of a small base station is to apply a cell selection offset (CSO) to the cell selection algorithm and thus steer more users to the small base station. This is illustrated in FIG. 1, wherein the small base station 110 have a relatively small coverage area constituting a pico cell 120 due to its low transmit power. A number of UEs 130a are connected to the macro site 140. Some UEs 130b will connect to the small base station 110, thereby offloading the traffic from the macro cells 150. By applying CSO, i.e. by biasing handover decisions between the different base stations such that some UEs 130c are handed over to the small base station earlier than usual, load is shifting from the macro site 140 to the small base station 110. As this corresponds to an expansion of the range of the small base station, this feature is typically referred to as “range expansion”.
One potential problem with applying a CSO to the cell selection algorithm is that users in the cell range extended (CRE) area 160, so called CRE users, might be interfered heavily from the macro site in the downlink. This results in a very low downlink Signal-to-Interference Ratio (SIR). This in turn may lead to that users in the CRE area will experience problems in demodulating control and/or data signals, leading to lower performance. One way to mitigate this is to apply muting restrictions in the macro layer. Using so called Almost Blank Subframes (ABS), macro cells are muted, in a static or dynamic way, in order to reduce interference to pico users in the CRE area.
By doing that, protection of both data and control channels of CRE users takes place.
There are mainly two problems with the ABS solution. Firstly, by muting the macro base station precious macro capacity is sacrificed in order to favor transmissions of the small base stations. Secondly, muting the macro base station only helps CRE users when the interference from the macro is what limits their performance. If a low signal strength from the own small base station in comparison with thermal noise is the limiting factor, it does not help to reduce macro interference. Furthermore, in the case of dynamic ABS coordinated scheduling between the macro site and the small base station is required in order for the macro site to be able to mute its transmission at the same time as the small base station schedules its CRE users.
Simulation results have shown that the loss of useful signal power for the CRE users is more important than the interference originating from the near-by macro site. In addition to that, simulations have shown that while increasing the CSO and steering more users to the small base station layer, macro interference becomes less and less important due to the fact that macro utilization drops.